The present invention relates to cable television systems and, in particular, to the retrofitting of existing cable television converters for use in accommodating stereo broadcast television sound.
The Federal Communications Commission has recently approved a standard for broadcasting stereophonic television sound. The standard approved utilizes the Zenith Broadcast Delivery System and the dbx noise-reduction system.
Cable television systems were not designed for stereo. When cable operators in the past desired to deliver stereo audio to their audiences, they bypassed the cable networks entirely and used a technique known as "FM simulcasting" wherein the audio portion of a television program was received on a viewer's separate stereo receiver. While this method is adequate for limited stereo broadcasting of a few channels, it does not provide an acceptable solution to the problem of receiving stereo broadcast signals through a cable network.
A major problem in accommodating stereo broadcast signals in existing cable networks is that the audio signals are carried on the cable together with video signals which are usually scrambled for security purposes. In the process of descrambling the video signals for viewing by an authorized subscriber, the stereo signal transmitted according to the Zenith/dbx technique is distorted. As a result, existing cable television converters are not compatible with the recently approved stereo broadcast television signals.
The inability of the currently installed base of millions of cable television converters to accommodate stereo reception is a substantial problem. It would be very costly for cable system operators to replace all of the converters currently in use with new converters specially adapted for use with stereo broadcast television signals. The magnitude of the problem is highlighted in an article entitled "Stereo Unready - Why Your Cable Box Is Hopelessly Mono-tonous", published in the December, 1984 issue of Video Magazine, volume VIII, Number 9, pgs. 117-118, 188-190. As the title of the article implies, it has been assumed that the current generation of cable television converters would have to be scrapped and replaced with new stereophonic converters. The present invention, however, provides an apparatus and method for stereophonic reproduction of stereo broadcast television sound using existing cable television converters. Simple modifications are made to each converter to enable the audio signals and several control signals to be tapped therefrom, combined, and input to a stereo adapter which provides left and right stereo outputs.
Past converters, such as those shown in U.S. Pat. Nos. 4,095,258 ("Sperber"); 4,145,717 ("Guif et al"); and 4,163,252 ("Mistry et al") provide descrambling of scrambled cable television signals but make no provision for stereo sound. Although Mistry et al suggest that a stereo demodulator chip can be used in conjunction with the audio circuits in their converter, the provision of such capability would require substantial modification of the converter circuitry. Mistry et al have not considered, nor do they make any suggestion as to how an existing base of cable television converters could be inexpensively modified for use in accommodating stereo broadcast television sound.